A magnetoresistance (MR) sensor is used in a read/write head to read magnetic fields on a recording medium of a magnetic storage device. An example is the read/write head of a computer hard disk or a magnetic recording tape drive. The read/write head is positioned closely adjacent to the recording medium, separated from the recording medium by an air bearing surface (ABS) that does not allow them to touch. A data bit is written onto an area of the recording medium using the writing portion of the read/write head by locally changing its magnetic state. That magnetic state is later sensed by the MR sensor to read the data bit.
Two known types of MR sensors are a giant magnetoresistance (GMR) sensor and a tunnel magnetoresistance (TMR) sensor. The general technical basis, construction, and operation of the GMR sensor are described, for example, in U.S. Pat. No. 5,436,778. The general technical basis, construction, and operation of the TMR sensor are described, for example, in U.S. Pat. No. 5,729,410. The disclosures of both patents are incorporated by reference in their entireties. These patents also describe the read/write heads and the magnetic storage systems.
In one type of MR sensor structure, typified by the exchange tab GMR sensor and the in-stack bias TMR sensor, the sensor structure having a free layer that responds to external magnetic fields is overlaid, at least in part, with an upper antiferromagnetic layer. The upper antiferromagnetic layer exchange couples to the portion of the free layer that is overlaid. A relatively thick upper ferromagnetic “soft” layer is placed between the upper antiferromagnetic and the free layer to provide some magnetostatic bias to the sensor. However, as the upper ferromagnetic layer is made thicker, the exchange coupling between the upper antiferromagnetic layer and the overlaid portion of the free layer is reduced. The reduction in exchange coupling leads to magnetic instability due to domain formation in the free layer. The reduced exchange coupling also causes side-reading problems, so that the sensor detects magnetic fields laterally separated from the magnetic track that is in registry with the sensor structure.
There is a need to overcome these problems, without losing the beneficial aspects of the MR sensor structure. The present invention fulfills this need, and further provides related advantages.